Review with your students what they have learned about the human eye and how it receives visual images. Tell the class that they are going to make models of the eye that may reveal to them something surprising about the way we see.

2.

Divide the class into groups, giving each group the materials listed previously (see Materials). (If you can obtain only one round glass bowl, let groups take turns using the materials.)

3.

Instruct students how to make their model eyes, or reproduce the following set of instructions and give a copy to each group:

Make a small pencil hole in the middle of the black cardboard.

Stand the black cardboard against one side of the bowl and the white cardboard on the other side, opposite the black.

Turn on the lamp (or light the candle), and place it so it is shining through the hole in the black cardboard.

Darken the room as completely as possible.

Move the white cardboard from side to side until an image of the lamp or candle appears on it.

4.

After students have created their models, explain how each part of the model corresponds to a part of the eye:

The hole in the black cardboard represents the pupil, a small hole in the front of the eyeball that lets light into the eye.

The round bowl of water represents the eyeball.

The white cardboard represents the retina, the part of the eye that receives images and sends them to the brain via the optic nerve.

5.

Ask students why the image they see on the “retina” is upside down. Explain that the curvature of the eyeball (or the round glass bowl) inverts the image by bending the light as it comes through. The image that forms on the retina in the back of the eye is also upside down, but the brain interprets the image so that it is seen right-side up.

6.

Have each student produce a labeled diagram of the model indicating which part of the eye each part of the model represents.

Explain the difference between light reflection and light refraction, and provide examples of each of these processes in action in everyday life.

2.

As scientists have made clear, light is a strange phenomenon. On some occasions, it acts as if it were made up of individual particles called photons. On other occasions, however, it acts as if it were made up of waves. Explain the difference between these ways of imagining light.

3.

Compare and contrast how a camera works with how the human eye works. List the mechanical parts that would be similar and the parts that would be different.

4.

Describe several different vision irregularities in humans and discuss the various methods that are used to correct them.

Just an IllusionStudents are often amazed at the effects that a simple glass of water can have on light. Water can actually distort images in some fairly astonishing ways. To begin this activity, ask your students to drop a penny into a clear, standard-sized beaker of water and then stare at the penny through the side of the beaker. If they move their heads around a bit, it won’t take much effort to make the penny seem much larger—almost the size of a silver dollar. Next, ask them to put a pen or pencil into the beaker, halfway out of the water, and then view it again from the side. The pencil will seem broken, as if the half below the water were unconnected to the half above it. Ask students to speculate about the reasons behind these optical illusions. (In the first case, the light reflecting off the penny diverges in the water before it reaches the students’ eyes, making the penny seem larger. In the second case, the light reflecting off the submerged half of the pencil is refracted, whereas the light on the dry half is not, making the image seem disjoined.)

How Thick Is Your Lens?As light travels through a convex lens, its waves are refracted toward a single focal point. The thickness of the lens determines where this focal point lies. Scientists often measure the focal length of a lens—the distance between the middle of the lens and its focal point. Thick lenses have shorter focal lengths than thin lenses. Divide your students into groups, and ask each group to determine the focal lengths of a variety of convex lenses—specifically, the lenses in any pairs of glasses worn by your students. (If few or none of your students wear glasses, you can still conduct the experiment using readily available laboratory lenses.) Have students hold a flashlight 2 meters from a comb, which itself should be held perpendicular to a piece of white paper. Students should first trace the shadows cast by the comb. Then, once the shadow lines are drawn, students should hold one lens—convex side toward the light—between the flashlight and the comb, adjusting its position between the two until the new shadows cast by the comb match the traced outline. When the shadows match, the group should measure the distance between the lens and the comb to determine its focal length. This process can be repeated for each subsequent lens. When their focal length data are complete, ask students to determine the relationship between lens thickness and focal length.

The Color of NaturePat Murphy and Paul Doherty. Chronicle Books, 1996.This unique book is written by a physicist who explains the science of colors. The glorious color photographs of all aspects of nature, from clouds and rainbows to flowing lava and the feathered wings of birds, inspire readers to examine more closely the science of color and light.

The Optics Book: Fun Experiments with Light, Vision, and ColorShar Levine and Leslie Johnstone. Sterling Publishing Company, 1998.Experiments involving light rays, the speed of light, color, vision, polarization, and optical instruments fill this exciting book. Every experiment includes a complete list of required materials, instructions, and a final piece explaining “what happened.” Color photographs, line drawings, and a glossary round out this work.

Optical Bench - A Virtual Applet LabPrint out a copy the Optics Bench worksheet and it will guide your students through an online interactive virtual lab that teaches the fundamentals of geometric optics for thin lenses.

Cow's Eye DissectionLearn about the anatomy of the eye with this step by step study guide on the dissection of a cow's eye provided by the Exploratorium.

Recycle a Potato Chip Can into a Pinhole Simple CameraConstruct the simple pin hole camera described at this website and compare its design to the anatomy and function of the parts of an eye. Think about and give examples of how inventions imitate what living things do naturally.

Definition:The sensory membrane that lines the eye is composed of several layers including one containing the rods and cones, and functions as the immediate instrument of vision by receiving the image formed by the lens and converting it into chemical and nervous signals that reach the brain by way of the optic nerve.Context:At the back of the eyeball is a thin, delicate, light-sensitive membrane called the retina, which is connected to the brain by the optic nerve.

This lesson plan may be used to address the academic standards listed below. These standards are drawn from Content Knowledge: A Compendium of Standards and Benchmarks for K-12 Education: 2nd Edition and have been provided courtesy of theMid-continent Research for Education and Learningin Aurora, Colorado.

Grade level:6-8, 9-12Subject area:scienceStandard:Understands the nature of scientific inquiry.Benchmarks:Benchmark 6-8:Designs and conducts a scientific investigation (e.g., formulates questions, designs and executes investigations, interprets data, synthesizes evidence into explanations, proposes alternative explanations for observations, critiques explanations and procedures).Benchmark 6-8:Knows that scientific inquiry includes evaluating results of scientific investigations, experiments, observations, theoretical and mathematical models, and explanations proposed by other scientists (e.g., reviewing experimental procedures, examining evidence, identifying faulty reasoning, identifying statements that go beyond the evidence, suggesting alternative explanations).Benchmark 6-8:Knows possible outcomes of scientific investigations (e.g., some may result in new ideas and phenomena for study; some may generate new methods or procedures for an investigation; some may result in the development of new technologies to improve the collection of data; some may lead to new investigations).Benchmark 9-12:Knows that scientists conduct investigations for a variety of reasons (e.g., to discover new aspects of the natural world, to explain recently observed phenomena, to test the conclusions of prior investigations, to test the predictions of current theories).Benchmark 9-12:Designs and conducts scientific investigations by formulating testable hypotheses; identifying and clarifying the method, controls, and variables; organizing and displaying data; revising methods and explanations; presenting the results; and receiving critical response from others.